The present invention relates generally to an apparatus and method for electrical discharge machining and, in particular, to an apparatus and method for electrical discharge machining which includes an improved work station that provides automatic alignment of the electrode to the workpiece.
Electrical discharge machining (EDM) is a well known process for forming features, such as holes, slots and notches of various shapes and configurations, in an electrically conductive workpiece. Conventional EDM apparatuses typically employ an electrode having the desired shape that is advanced toward the workpiece. A suitable power supply is applied to create an electrical potential between the workpiece and electrode for forming a controlled spark which melts and vaporizes the workpiece material to form the desired feature. The cutting pattern of the electrode is usually computer numerically controlled (CNC) whereby servomotors control the relative positions of the electrode and workpiece. During machining, the electrode and workpiece are immersed in a dielectric fluid, which provides insulation against premature spark discharge, cools the machined area, and flushes away the removed material.
One drawback to electrical discharge machining is that it is a relatively slow process, especially when several distinct features need to be machined into a workpiece. This is particularly so in the aircraft engine industry where electrical discharge machining is widely used for machining various features into aircraft engine parts. To increase the manufacturing output of such parts, it is common to use an EDM apparatus that machines a number of parts at one time. Such an apparatus has a plurality of work stations, each of which has a workpiece fixture located in a single dielectric tank. The work stations are all typically connected to a common power supply. Thus, machining takes place in series one part at a time. That is, a spark will be created in the first work station and then the next work station and so on until each station has a spark supplied. This sequence is repeated until the machining operation is completed for each workpiece.
A problem with this type of apparatus is that whenever one station gets hung up (i.e., fails to discharge for some reason such as electrode misalignment or a EDM particle remaining in the electrode-workpiece gap) all of the stations will become hung up. Since all stations are stopped, it is not evident which station is causing the stoppage. Furthermore, each station must use the same electrode material and polarity because of the series power connection. Thus, each station machines the same feature into the parts. Parts requiring additional features must then be moved to another machine. This means that multiple machines, fixtures and part handling are required before a part is completed Another inefficiency with these conventional EDM apparatuses is that the dielectric tank must be drained and refilled between each cycle to load and unload the parts because the workpiece fixtures are all located and submerged in the tank.
Various approaches to solving such problems are disclosed in U.S. Pat. No. 6,326,576 to Krenz et al., U.S. Pat. No. 6,369,343 to Krenz et al., and U.S. Pat. No. 6,563,071 to Krenz. While each EDM apparatus disclosed in such patents is useful for its intended purpose, it will be appreciated that the slide mechanism which houses the electrode is indirectly connected to a base supporting the workpiece. Accordingly, the slide mechanism moves along a plurality of ceramic rods into position with respect to a workpiece. In this way, the electrode is electrically insulated from the fixtures retaining the workpiece. Not only do such ceramic rods and its related components increase the bulkiness of the slide mechanism, this type of configuration also produces stack-up errors which require manual alignment of the electrode to each workpiece by the machine operator. This introduces a window of error if the electrode is incorrectly aligned, as well as a measure of inefficiency to the process.
Accordingly, it would be desirable for an electrical discharge machining apparatus to be developed which includes a work station which automatically aligns an electrode with a workpiece so as to form a desired feature. It would also be desirable for the work station of such EDM apparatus to be greatly reduced in size and complexity so that multiple features can more easily be formed in a workpiece.